1. Field of the Invention
This invention relates to information signal reproducing apparatus and more particularly to such type of apparatus which reproduces information signal from a recording medium having formed thereon a number of tracks in which four pilot signals having different frequencies from one another are recorded successively one at each track along with information signals.
2. Description of the Prior Art
A magnetic tape may have successive cross tracks formed in sequence by rotating two heads during recording of video signals, and a magnetic video reproducing apparatus (hereinafter called "VTR") may reproduce the video signals from such tracks. The following may, for example, involve such tape.
Two tracking control methods have been employed for precisely tracking VTR recording tracks during reproduction.
In the first method, control signals which have been recorded in synchronism with video signals are used for obtaining a tracking control signal during reproduction. The second method involves superimposing a prescribed pilot signal on the video signals during recording. For reproduction, this pilot signal is processed to obtain a tracking control signal.
The present invention can be applied to great advantage in VTRs employing the aforesaid second tracking control method and, particularly, in VTRs in which four pilot signals of different frequency are successively superimposed on video signals, with one of the four pilot signals on each track.
The basic concept of this type of VTR is explained with reference to FIG. 1. Four pilot signals (f1, f2, f3, f4) are recorded on respective tracks, in the order of f1, f2, f4 and f3, in superimposed relation on video signals. For reproduction, a signal having the same frequency as the pilot signal superimposingly recorded on those tracks to be principally treated, is multiplied to obtain crosstalk components from the front and rear adjacent tracks. Tracking is controlled by comparing their levels with each other.
The use of this method makes it possible to obtain a tracking signal throughout the entire length of the video track. With a video head attached on a bimorph or other suitable electric-mechanic converting element, precise tracking is assured and clear images free from noise can be reproduced at a desired speed, even when the tape is transported at a different speed during reproduction from the speed during recording (variable-speed reproduction). For variable-speed reproduction, however, the reproduced pilot signal must be multiplied by a signal of the same frequency as that of the pilot signal superimposed on the video signal in the principal track solely to be traced by the video head.
In FIG. 1, a reproducing mode which operates at, for example, three-times the speed of recording is illustrated. Tracks Am and Bm (m=integer) arc recorded on the magnetic tape by the rotating two-head helical scan type VTR for heads A and B having magnetizing directions different from each other. In FIG. 1, V indicates an area from which the video signals are reproduced between overlapping areas O1 and O2 and f1, f2, f3 and f4 on the respective track lines represent the frequencies of the pilot signals superimposed on the video signals in the corresponding tracks.
The head A first traces a track A1. Subsequently, the head B traces another track B2. The head A then transfers to a track A4, the head B to a track B5, and so on. In such track-tracing, the pilot signals reproduced in sequence are f1.fwdarw.f3.fwdarw.f4.fwdarw.f2 (.fwdarw.f1 . . . ) for each field, which is different from that in the normal reproduction mode, or the normal pilot signal rotation (f1.fwdarw.f2.fwdarw.f4.fwdarw.f3). This rotation changes at various tape speeds multiplied by integers during reproduction as summarized in Table 1 below.
TABLE 1 ______________________________________ Speed Factor (n: Integer) Pilot Signal Rotation ______________________________________ 4n f1 .fwdarw. f2 or f2 .fwdarw. f4 or f4 .fwdarw. f3 or f3 .fwdarw. f1 4n + 1 f1 .fwdarw. f2 .fwdarw. f4 .fwdarw. f3 4n + 2 f1 .fwdarw. f2 or f2 .fwdarw. f4 or f4 .fwdarw. f3 or f3 .fwdarw. f1 4n + 3 f1 .fwdarw. f3 .fwdarw. f4 .fwdarw. f2 ______________________________________
The varying speed reproduction modes are not limited to the cases where the tape running speed is multiplied by integer values, but may include slow motion reproduction and other cases where the factor by which the tape running speed is decreased or increased does not have an integer value. In such cases, the rotation of the pilot signals reproduced becomes further complicated.
FIG. 2 illustrates a circuit for rotation of the pilot signals during recording. Table 2 is a function table for the data selector 6 of FIG. 2. FIGS. 3(A) and 3(B) are pulse timing charts of wave-forms in the lines A and B of FIG. 2, respectively. In FIG. 2, as a cylinder 1 rotates, a phase detector head 2 picks up a signal (cylinder PG). The output of head 2 is delayed by a mono-stable multivibrator 3 and is formed by a flip-flop (FF) 4 into a head selection pulse (so called 30 PG), which is applied as a select input A to a data selector 6. Another FF 5 acts as a frequency divider which receives the 30 PG to produce an output which is applied as another select input B to the data selector 6. The data selector 6 operates as shown in Table 2. If both the input A and the input B are high, the signal f4 from an input line 14 is placed onto an output line 15; if the input A is low while the input B is high, the signal f3; if the input A is high while the input B is low, the signal f1; and if both inputs A and B are low, the signal f2, is selectively placed onto the output line 15. Therefore, the pilot signals appear in the order of f1.fwdarw.f2.fwdarw.f4.fwdarw.f3, as indicated in FIGS. 3(A) and 3(B).
TABLE 2 ______________________________________ Operation of the data selector Control Input Data Input Output A B f1 f4 f2 f3 Y ______________________________________ H L H -- -- -- H H H -- H -- -- H L L -- -- H -- H L H -- -- -- H H ______________________________________
Such a pilot signal forming circuit has found general use in obtaining multiplier signals by which the reproduced pilot signals are multiplied at the time of reproduction. During operation in a normal reproduction mode, this circuit can be used for recording and reproduction. However, when varying speed reproduction is preformed, the rotation of the multiplier signals must be altered, depending on the value of the tape running speed. To allow the tape speed to increase, for example, by a factor of any integer, the pilot signal forming circuit must be constructed to produce the pilot signals in one of the rotations shown in Table 1 by taking into account the value of the tape speed. This requirement cannot be fulfilled without involving a great increase in the complexity of structure of the signal forming circuit. Also, it has been impossible to carry out the operation of changing the aforesaid speed magnification in analog form.